Golf balls are typically divided into two general types or groups: a) solid golf balls and b) wound balls. The difference in play characteristics resulting from these different types of constructions can be quite significant. Solid golf balls may be one-piece or multilayer, while wound balls always have at least three layers of at least a center, wound layer, and a cover.
Solid golf balls having a two-piece construction are generally most popular with the recreational golfer because they provide a durable ball capable of achieving long distances in play. Two-piece balls are typically made with a single solid core, usually formed of a crosslinked rubber, which is encased by a cover material. Typically the solid core is made of polybutadiene chemically crosslinked with a metal salt of an unsaturated fatty acid and/or similar crosslinking agents. Covers typically are ionomers and blends of ionomers with other thermoplastics, such as SURLYN.RTM. resins, which are ionomer resins sold commercially by E. I. DuPont de Nemours of Wilmington, Del. or IOTEK.RTM., which is sold commercially by Exxon Corporation.
Solid golf balls having three or more layers arc more expensive to produce, but often provide improved playing characteristics. Such balls typically have a rubber-based spherical center around which is concentrically disposed one or more intermediate layers, also known as mantle layers. One or more cover layers are typically disposed concentrically about the intermediate layer(s) similar to two-piece golf balls.
Wound balls typically have either a solid or liquid filled center around which a tensioned elastomeric material (i.e., a stretched elastic thread) is wound to form a core. The wound core is then covered with a cover material, which may be the same as those discussed above for solid golf balls but also typically include balata (trans-polyisoprene) and urethanes. However, like three-piece multilayer solid golf balls, the more complex structure of wound balls generally results in a longer manufacturing time and greater expense in the production thereof compared to a two-piece ball.
One difficulty common to preparing solid multilayer balls is that materials of an outer layer do not necessarily bond well with the materials used in the inner layer(s). This can result in layer separation, particularly when the golf ball is struck by a club, which can detrimentally affect the playability and appearance of the golf ball. Moreover, should the cover be cut or damaged, improper bonding between layers tends to permit further degradation of the cover or even complete disintegration of the ball layers.
Various types of surface treatment techniques are known for use in modifying polymer surfaces. These techniques include mechanical abrasion; chemical abrasion, such as etching; and high-voltage electrostatic discharge, also known as corona treatment. See, e.g., U.S. Pat. No. 5,466,424 (corona discharge surface treating method) and Stobbe, Bruce, "Corona Treatment 101," Label and Narrow Web Indus., May-June, 1996.
One method of modifying polymer surfaces is plasma treating. Plasma treatment of various shapes and types of polymers in general is well known. See, e.g., Kaplan, S. L., "Cold Gas Plasma Treatment for Re-Engineering Films," Paper Film Foil Converter, 71(6) June, 1997; Rose, P., et al., "Treating Plastic Surfaces with Cold Gas Plasmas," Plastics Engineering, pp.41-45 (October, 1985). Plasma treatment generally oxidizes the surface of a material being treated. For example, U.S. Pat. No. 5,387,842 discloses a steady-state, glow discharge plasma generated within the volume between a pair of parallel, insulated metal plate electrodes spaced up to 5 cm apart and RF energized with an rms potential of 1 to 5 KV at 1 to 100 KHz. The electrodes are disclosed to be located within an enclosure capable of maintaining an atmosphere other than atmospheric air, such as a noble gas, between the electrode surfaces. See also U.S. Pat. No. 5,316,739 and U.S. Pat. No. 5,098,483 (methods of treating spherical surfaces).
U.S. Pat. No. 5,414,324 discloses a similar parallel plate apparatus and process, but charges the electrodes with an impedance matching network adjusted to produce a stable, uniform glow discharge at atmospheric pressure, which is also known as corona discharge.
U.S. Pat. Nos. 5,403,453 and 5,456,972 disclose polymer materials, such as film and fabrics, that may be non-destructively surface treated to improve water wettability by exposure to a glow discharge plasma sustained at substantially atmospheric pressure in a modified gas atmosphere of helium or argon.
U.S. Pat. No. 4,919,434 discloses a golf ball having a cover which includes an inner cover layer and an outer cover layer, each of which includes a thermoplastic resin. Preferably, the layers are formed of materials capable of fusion bonding with each other to properly adhere the layers together.
JP Patent Document No. 60215374 discloses a golf ball made of synthetic resin, wherein the golf ball cover surface is subjected to microwave plasma treatment to improve the thin coating layer's peeling resistance against shock or flexing fatigue. This process is disclosed as greatly reducing treatment time from 10 to 24 hours down to several minutes.
U.S. Pat. No. 4,613,403 discloses a method for treating a golf ball surface made of balata resin, thermoplastic elastomer, ionomer resin, or the like with unpolymerizable gas plasma and subsequently applying the usual coating in an apparatus adapted to expose the surface of the ball to the plasma. The process is alleged to make the coating on the golf ball hard-exfoliative, i.e., more mar-resistant.
U.S. Pat. No. 5,286,532 discloses a method for producing golf balls by surface-treating the golf ball with atmospheric pressure plasma prior to finish coating to provide a good adhesion of the coating to the golf ball, which coating is highly resistant to discoloration and deterioration.
However, these references do not disclose methods for treating one or more internal layers within a multilayer ball, i.e., other than on the surface, to provide golf balls having improved durability and acceptable playing characteristics, such as low driver spin and high initial velocity. It is thus desirable to provide a method capable of producing such multilayer golf balls, which golf balls themselves are also novel.